This invention pertains to regeneration of used supported noble metal catalysts. It pertains particularly to regenerating used supported noble metal catalysts such as palladium catalysts utilized for hydrogenation of ethyl anthraquinone (EAQ) for producing hydrogen peroxide (H2O2) product, so as to achieve regenerated catalyst activity levels near or exceeding that of the fresh catalyst.
The conventional production of hydrogen peroxide product involves a two-step process in which a hydrogen donor solvent ethyl anthraquinone (EAQ) is first hydrogenated and then oxidized with oxygen to form the hydrogen peroxide product. In some hydrogen peroxide manufacturing facilities, the hydrogenation step is carried out in a fixed bed reactor utilizing a palladium/alumina or similar catalyst. A typical such catalyst may contain 0.28 to 0.33 wt % palladium on a large pore alumina support. The useful life expectancy of the catalyst is about two years, after which its activity drops to about 30% of its original (fresh catalyst activity) condition or level. It is believed that this catalyst deactivation is caused by deposition of high molecular weight organic materials formed from the polymerization of EAQ on the active sites of the catalyst, and/or by gradual agglomeration of the palladium to larger particles or clusters on the catalyst. Such used or spent palladium/alumina catalysts are presently being regenerated using a simple xe2x80x9cwash-burnxe2x80x9d procedure, in which the catalyst is first extracted with an organic solvent to remove any soluble material deposits, and then subjected to a controlled carbon burn-out step at about 850xc2x0 F. temperature in air. Such used or high temperature regeneration treatment may also promote undesirable agglomeration of the palladium to larger particles on the catalyst support. Thus, it is difficult to successfully regenerate the used catalyst back to near 100% of its original activity. In fact, this simple wash-burn procedure can usually restore the used catalyst to only about 70% of its original or fresh activity level. Such xe2x80x9cwash-burnxe2x80x9d catalyst regeneration procedures have been disclosed by various U.S. and foreign patents. For example, U.S. Pat. No. 4,148,750 to Pine discloses a process for redispersal of noble metals on used supported zeolite-containing catalysts. U.S. Pat. Nos. 4,454,240 and 4,595,666 to Ganguli disclose method steps for regenerating used catalysts by dilute acid treatment to remove undesired metal deposits followed by carbon burn-off at increased temperature levels. Also, U.S. Pat. No. 5,188,996 to Huang et al discloses redispersion of noble metal such as platinum on low acidity support such as silica by contacting with chlorine and oxygen at low pressures.
It is desired to improve economics of the hydrogen peroxide production process by increasing the activity and service life of the palladium/alumina catalyst, as well as that of other similar supported noble metal catalysts. Based on an understanding at molecular level of the apparent catalyst reaction and deactivation mechanism, the surface structure of the catalyst support material, and the exposition of palladium crystal clusters thereon, an effective procedure has been developed for regenerating and enhancing used palladium/alumina catalyst to an activity level significantly higher (90% or more of fresh catalyst activity) than that achieved by the current xe2x80x9cwash-burnxe2x80x9d procedure.
This invention provides a regeneration method and procedure for regenerating and enhancing used supported noble metal catalysts. For the fresh catalyst, the support material such as alumina (AA) or silica (AN) having an initial surface area of 20-600 m2/gm, surface area and 100-400 Angstroms pore diameter being preferred. The noble metals provided on and in pores of the support include palladium (Pd), platinum (Pt), gold (Au), iridium (Ir), osmium (Os) rhodium (Rh), or ruthenium (Ru), or combinations thereof, with palladium usually being preferred. The invention is particularly useful for regenerating and enhancing used supported palladium (Pd) catalyst, such as utilized for hydrogenation of ethyl-anthraquinone (EAQ) for producing hydrogen peroxide (H2O2) product.
The used catalyst regeneration method of this invention includes the following three basic steps:
(1) Cleaning the used supported noble metal catalyst by solvent extraction for removal of process contaminants and adsorbed chemicals from the used catalyst by contact with suitable organic solvent(s);
(2) Drying and calcining the cleaned used catalyst to remove any process contaminants or polymer deposits; remaining on the catalyst after the solvent extraction;
(3) Contacting the cleaned and calcined catalyst with a suitable organic treating agent selected for forming an organo-metallic complex for breaking down large noble metal agglomerates or clusters on the used catalyst to smaller metal particles, and redistributing the smaller noble metal(s) particles on or within the pores of the catalyst support. Suitable catalyst treating agents should have an ionization constant pK1 greater than about 2.5.
The organic solvents suitable for the used catalyst cleaning method step (1) by solvent extraction of the used supported noble metal catalyst can be alcohols such as methanol, amines, ketones, or similar organic compounds utilized at cleaning conditions of 0-200xc2x0 C. temperature and 1-50 atm. pressure for 2-8 hours contact time. Preferred solvent cleaning conditions are 10-100xc2x0 C. temperature and 1-20 atm. pressure for 4-6 hours contact time. Suitable catalyst drying and calcining conditions for the method step (2) are heating the catalyst in air at 100-120xc2x0 C. for 1-8 hours for the drying step, then further heating the dried catalyst in air at 200-600xc2x0 C. (392-1112xc2x0 F.) temperature for 1-24 hours for the calcining step. Preferred calcining conditions are 250-500xc2x0 C. temperature and ambient pressure for 2-10 hours duration, Calcining the dried catalyst at lower temperatures and longer time periods within these ranges is usually preferred for economic reasons.
The organo-metallic complex forming chemical treating agents suitable for the noble metal redistribution method step (3) on the cleaned noble metal catalysts are chemical compounds having carbon atoms not exceeding about 20 and molecular weight not exceeding about 300. The treating agent should also have an ionization constant pK1 greater than about 2.5, as defined by the following equation and its transformation:
RCOOH+H2O⇄RCOOxe2x88x92+H3O+
      K    1    =                    [                  RCOO          -                ]            ⁡              [                              H            3                    ⁢                      O            +                          ]                    [      RCOOH      ]      
Some examples of organo-metallic complex forming chemical treating agents and their corresponding ionization constants pK1 are as follows:
Oxalic acid having pK1 of 1.27 and ethylene diaminetetraacetic acid (EDTA) having pK1 of 2.01 are outside this desired range, and are thereby not suitable for providing desired organo-metallic complexes for this invention. Also, EDTA has been shown to remove aluminum from zeolites by a chelation effect which can thereby render zeolite supports ineffective by deactivating the support. Useful reaction conditions for forming the organo-metallic complexes and for redistributing the noble metal particles from the clusters on the catalyst support by the treating agent(s) are 10-500xc2x0 C. temperature and 1-10 atm. pressure for 1-8 hours duration, with 20-450xc2x0 C. temperature and 1-5 atm. pressure for 2-6 hours duration being preferred. For best results, the organic treating agent should preferably be maintained in its liquid phase, however a liquid/vapor phase mixture having only a small portion of vapor may be utilized.
By utilizing the used catalyst regeneration method and procedure according to this invention, it is proved experimentally that used palladium (Pd) catalyst supported on alumina can be better cleaned for removal of the process contaminants and polymer deposits, and thus expose more catalyst surface and active Pd sites to a process reactant. Catalyst activity tests have also shown that used supported Pd catalysts regenerated by the method of this invention have their activity significantly increased to at least about 80% and preferably up to 93%-103% of fresh catalyst activity level, compared to only about 70% of fresh catalyst activity after being regenerated by known traditional xe2x80x9cwash-burnxe2x80x9d regeneration procedures. This regeneration method is particularly useful for used supported catalysts containing 0.2-0.4 wt. % palladium deposited on a support of alumina or silica, as utilized for producing hydrogen peroxide (H2O2) by hydrogenation of ethyl-anthraquinone (EAQ).
Advantages provided by the catalyst regeneration method and procedure of this invention include its ability to not only effectively remove contaminants and organic deposits from the used noble metal catalyst, but also to break apart and redistribute the active noble metal molecules such as palladium in the pores of the catalyst support. This new catalyst regeneration method and procedure not only cleans the contaminated catalyst surface, but also improves the exposition and distribution of the noble metal(s) such as palladium on the catalyst support. The regeneration procedure can restore catalyst activity to 100% or more of the fresh catalyst standard, and the resulting molar selectivity ratio of desired product to side products is 190:1, which is a better molar selectivity ratio than that achieved for fresh catalyst (150:1). This catalyst regeneration procedure and method is considered useful for regenerating supported noble metal catalysts containing other noble metals instead or in addition to palladium. After such used catalyst regeneration, the process reactant(s) have improved contact with the catalyst active metal(s) particles and thereby enhance the activity and product selectivity of the catalyst.